Water based photoresists based on natural, sensitizable protein products such as fish glue, albumin ad casein have been known for a number of years. A useful photoresist composition comprises casein, make by acid precipitation of milk, an alkali metal base to impart a pH of 7.7 or higher which improves the solubility of the natural product in water, and an ammonium dichromate sensitizer.
U.S. Pat. No. 4,061,529 issued on Dec. 6, 1977 to Goldman and Datta, herein incorporated by reference, discloses the addition of sodium borate as the base in a concentration so that the resist solution has a final pH of 6.7-7.3, to improve the pot life of the coating composition and the shelf life of coated substrates.
These resists have been employed extensively in the lithographic printing arts and in the manufacture of shadow masks for color television picture tubes. A cleaned metal surface or substrate is coated with the aqueous photoresist and dried. A master having a desired pattern is contacted to the resist and exposed to ultraviolet light which hardens the resist in the exposed areas. The resist film is then washed with water to dissolve the unexposed resist and thereby uncover part of the metal substrate. The now partially coated substrate is then dried and baked for about 5 minutes at a temperature of from 500.degree.-550.degree. F. (260.degree.-287.degree. C.). This step is required to render the remaining photoresist etch resistant. The partially coated substrate is then etched by spraying with hot ferric chloride solution which etches away the bared metal portions. The residual resist can then be removed in a hot alkali solution.
The above process has been universally adopted for television shadow mask production because of its low cost, e.g. the photoresist, water rinse and caustic wash solutions are very inexpensive as compared to organic based photoresists.
The photoresist formulation described in U.S Pat. No. 4,061,529 typically contains 17-20 weight percent borax (hydrated sodium borate) and about 22 weight percent ammonium dichromate. The patentees believe that the borax alkalizing agent solubilizes the casein without degrading it and further retards the curing of the photoresist composition in the absence of light thereby improving shelf life.
While the above-described photoresist formulation is satisfactory for its intended purposes, further improvements in the formulation can be achieved by utilizing a different alkalizing agent, reducing the amount of the dichromate sensitizer and reducing the pH of the final resist composition within the range of about 6.0 to 7.0. The present novel method of making a borax-free resist formulation provides a film that has improved visual uniformity since the elimination of borax, which makes no positive photochemical contribution to the resist composition, removes some impurities from the photoresist. The present borax-free formulation also has increased both photosensitivity (borax served as an inhibitor during exposure), and shelf life by reducing the dichromate concentration over that disclosed in U.S. Pat. No. 4,061,529. The borax-free resist is less expensive than the resist of U.S. Pat. No. 4,061,529 since borax has been eliminated and replaced with a lower concentration of sodium hydroxide, also the dichromate concentration has been reduced and the concentration of casein has remained unchanged.